The present invention relates in general to an apparatus for electrostatic spray coating of liquid or dry charged particles onto workpieces at a coating station such as a paint spray booth and, more particularly, to a workpiece fixture in the nature of hangers or tiered support members which are fabricated of metal and used for supporting plural articles as they are conveyed through a finishing system.
Workpiece fixtures are frequently used to support a plurality of articles while they are being surface treated. These fixtures are usually provided with several vertically stacked, horizontal rows of protruding article-engaging hooks upon which workers hang articles to be finished. Ordinarily, each hook is constructed of a piece of wire having its article-engaging end bent into a hook shape. However, it should be understood that the term "hook" as utilized herein includes the great variety of configurations which may be devised for engaging parts to be finished. The workpiece fixture and its suspended articles travel through the finishing system which can have a series of stations at which they may be sequentially treated with one or more of the steps of cleaning, rinsing, surface preparation such as undercoating, electrostatic spray coating and baking.
In electrostatic spray coating operations, it is necessary that the workpiece fixture establish and maintain electrical continuity between the suspended articles and an overhead carrier or conveyor, in order that the articles to be coated can be electrically charged so as to attract the oppositely charged coating material. It is therefore important that electrical continuity between each article-engaging hook and the frame of the workpiece fixture be maintained as a good, low resistance electrical contact. The accumulation of non-conductive or high resistance coating material at the interfacing electrical contacting surfaces between each article-engaging hook and the frame must be prevented. Unfortunately, repetitive use of the workpiece fixture results in a build-up of coating material between these interfacing electrical contacting surfaces with the eventual loss of electrical continuity with the suspended articles. Eventually, this build-up must be removed from the workpiece fixture, usually by immersing and soaking the fixture in a solvent bath for a prolonged period of time. The need for frequent cleaning demands either prolonged downtime of the coating apparatus or a large inventory of workpiece fixtures, both of which add materially to the cost of the coating operation.
One known workpiece fixture is disclosed in U.S. Pat. No. 4,217,853. A disadvantage with this known workpiece fixture is the difficulty of removing the article-engaging hooks from the main frame of the fixture, especially when encrusted with a build-up of coating material. This encrustation renders it difficult, if not impossible, to substitute article-engaging hooks of a different size or to replace those which become broken. In addition, once the main frame has been encrusted with coating material, it is relatively difficult during replacement of the article-engaging hooks to insure good, low resistance electrical continuity between the interfacing electrical contacting surfaces. Another disadvantage is that this workpiece fixture is relatively expensive to construct because it requires a substantial quantity of metal and numerous manufacturing operations.